Mr Brown's task now is to convince Aucklanders that the study is robust and its conclusions are right. Photo / Paul Estcourt

The reactions of Len Brown and Gerry Brownlee when the City Centre Future Access Study was released made it seem barely conceivable they had read the same report.

Auckland's Mayor hailed the study as a strong basis for entering funding negotiations for an inner-city rail loop. For the Minister of Transport, however, the report fell "some way short of convincing the Government it should provide financial support to any fast-tracking of the proposed city rail link". In effect, that implies a case of "as you were" for Mr Brown. If he wants work to start on the project quickly, he must convince Aucklanders to stump up the required $2.86 billion through the likes of road tolls, city parking levies and special rates.

It is in this context that the study by consultant Sinclair Knight Merz may have its biggest influence. It concludes that the inner-city loop, a 3.5km rail tunnel from Britomart to Mt Eden, would be no panacea for Auckland's congestion woes.

But it is comfortably the best option of the three short-listed. It should, says the study, be built by 2021 because delays would limit employment, growth and the ability to gain economic benefits.

Of the other options, a sole reliance on improved bus services would reach its limit between 2025 and 2030. It would mean that by 2021, peak-hour morning traffic would move at just 5km/h in the city centre, compared with the current 16km/h. The bus option would cost up to $1.1 billion and require a lot of residential and commercial property purchases for priority lanes. The third option, underground bus, provided only marginally more capacity than surface bus and would cost as much as the rail loop.

Even the inner-city rail loop would, according to the study, see morning traffic slow to 8km/h by 2021. But that provides the highest speeds for cars within the city centre, and is also the only option to deliver increased capacity beyond 2030.

But given its shortcomings, the study says the best way to ensure enough people can be brought into central Auckland to fill the extra jobs associated with its growth would be to build the rail loop while also improving the bus service. This would have a price tag of $3.3 billion and, because of extra buses, slow morning traffic entering the city to 6km/h by 2021.

Mr Brownlee objects to the study's conclusions on two grounds. First, he says, an assumption of central city job growth of 46 per cent in the next 10 years, compared with an increase of just 18 per cent in the past decade, is too ambitious. Auckland, however, has routinely been plagued by under-estimations of its growth. Secondly, the minister says the study underplays the role of highway improvements, such as the completion of the western ring route in 2017, which will draw many thousands of traffic movements away from the central city.

Given that criticism, it is significant that the study is not simply the work of Sinclair Knight Merz. Senior officials from the Ministry of Transport, the Transport Agency and the Treasury were involved. Further input was provided by Auckland Transport, which commissioned the study after the Government and the council had arrived at very different conclusions about the rail loop's return on every dollar invested. Given all that, this study should be the definitive research, not yet another document destined to gather dust. Mr Brown's task now is to convince Aucklanders that the study is robust and its conclusions are right. If he can, the Government should stand to one side.